In recent years, white light sources including light emitting diodes (LEDs) have attracted attention in terms of energy saving measures and a reduction in the amount of emitted carbon dioxide. As compared with conventional incandescent bulbs including tungsten filaments, LEDs have longer service life, and enable energy saving. As disclosed in Japanese Patent Laid-Open (Unexamined Publication) No. HEI 10 (1990)-242513 (Patent Document 1), in conventional white LEDs, YAG phosphors are excited using blue LEDs each having a light emission peak wavelength in a range of 400 to 530 nm, and the blue light emitted from the LEDs and the yellow light emitted from the YAG phosphors are mixed with each other, whereby white light is achieved and realized.
White light sources including LEDs have been widely used as backlights of traffic signal lights and liquid crystal displays (LCD) as well as general lighting equipment (illuminating equipment) such as room lights. In the light emission spectra of conventional white light sources including blue LEDs, the peak height of the blue light emitted from the blue LEDs is as large as at least 1.5 times the peak height of the yellow light emitted from phosphors, and hence influences of the blue light tend to be strong.
Under the circumstances, with the popularization of the white light sources including LEDs, adverse effects of the white light sources on human bodies start to be worried about. As described above, the light emission peaks of blue LEDs are strong in conventional white LEDs. Such white light having a strong blue emission peak is significantly different from that of natural light. Here, the natural light refers to sunlight.
According to the pamphlet of International Publication No. WO 2008/069101 (Patent Document 2), which has been achieved in consideration of the influences of such white light sources on human bodies, LEDs and phosphors having different light emission peaks are combined, and four types of light emission peak are thus mixed, whereby a white light with a small deviation from the spectral luminous efficiency is provided.
Here, the sensitivity of a human eye to light is referred to as luminosity function, and the spectral luminous efficiency is defined as standard spectral luminosity function V(λ) by International Commission on Illumination (CIE). Accordingly, the spectral luminous efficiency and the standard spectral luminosity function V(λ) are the same in meaning. FIG. 1 shows the spectral luminous efficiency V(λ) defined by CIE. That is, FIG. 1 shows that humans recognize light having a wavelength of about 555 nm at the highest sensitivity.
On the other hand, Patent Document 2 has an object to control light having a wavelength in a range of 420 to 490 nm, in consideration of influences of blue light on human bodies. Such a method can be expected to produce an effect of normalizing the secretion of melatonin that is one of hormones concerning adjustment by a biological clock in the nighttime.
In this regard, humans have a circadian rhythm (24-hour rhythm) controlled by an internal body clock. Humans are supposed to basically live under natural light, but there are a variety of lifestyles, such as long-time indoor work and a day-night reversal style, in modern society. If a life without exposure to natural light is continued for a long period, the circadian rhythm is disturbed, and adverse effects on human bodies are worried about.